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A065800 Numbers n which, for some r, are r-digit maximizers of n/phi(n). 0
6, 30, 60, 90, 210, 420, 630, 840, 2310, 4620, 6930, 9240, 25410, 50820, 76230 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

I can show that for r > 1, the first r-digit term of the sequence is the smallest r-digit primorial, if it exists. It remains to investigate the first terms when existence fails. It is also not hard to see that for r > 1, the r-digit terms are in arithmetic progression with common difference equal to the smallest r-digit term. For example, 210, 420, 630, 840 are in arithmetic progression with common difference 210. Obviously the r-digit minimizer of n/phi(n) is the largest prime of n digits.

LINKS

Table of n, a(n) for n=1..15.

EXAMPLE

30/phi(30) = 15/4 is maximal for two-digit numbers. 210/phi(210) = 35/8 is maximal for three-digit numbers.

CROSSREFS

Cf. A000010, A002110.

Sequence in context: A057229 A120734 A116360 * A181827 A263573 A145010

Adjacent sequences:  A065797 A065798 A065799 * A065801 A065802 A065803

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Dec 05 2001

STATUS

approved

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Last modified January 27 08:31 EST 2020. Contains 331293 sequences. (Running on oeis4.)